This invention relates to mercury thallium fluorophosphate glasses having unique physical characteristics. In particular, the glasses have very low transition temperatures (T.sub.g), very high refractive indices and dielectric constants, and are resistant to moisture attack. A low transition temperature is characteristic of a soft glass which may be useful in intricate molding, as well as sealing, operations.
At one time, the lead borate and lead borosilicate composition fields were studied intensively in a search for soft sealing and coating glasses. Later studies centered in the phosphate glasses, but durability and devitrification problems made these studies largely unfruitful.
Chalcogenide glasses have also been carefully investigated. These glasses prove very useful in specific applications, but do not normally have characteristics suitable for applications, such as molded articles or coatings, that are exposed to ambient conditions.
U.S. Pat. No. 4,285,730 (Sanford and Tick) discloses a group of glasses having compositions within the alkali metal oxide-alkaline earth metal oxide-fluoride-phosphate field (R.sub.2 O-RO-F-P.sub.2 O.sub.5) displaying low transition temperatures (T.sub.g) and the capability for molding and otherwise shaping glass bodies under pressure at low temperatures, viz., below 450.degree. C., without the need for hydration. Those glasses have base compositions consisting essentially, expressed in mole percent on the oxide basis as calculated from the batch, of about 15-45% R.sub.2 O, consisting of 0-45% Li.sub.2 O, 0-20% Na.sub.2 O, and 0-10% K.sub.2 O, 0-20% RO, consisting of 0-10% MgO, 0-15% CaO, 0-20% SrO, and 0-20% BaO, 25-55% P.sub.2 O.sub.5, with 0.3-3% by weight F, as analyzed in the final glass. Optional constituents therefor include up to 20% PbO, up to 7% La.sub.2 O.sub.3, and up to 12% ZnO.
United States application Ser. No. 327,915, now U.S. Pat. No. 4,362,819 filed Dec. 7, 1981 in the names of A. R. Olszewski, L. M. Sanford and P. A. Tick, discloses a family or alkali metal oxide aluminofluorophosphate glasses. These glasses have a transition temperature (T.sub.g) below 350.degree. C., thus permitting molding below 400.degree. C., and a defined resistance to moisture attack and weathering. They consist essentially, as analyzed on an atomic basis, of R'AlP.sub.2.0 F.sub.2.0 O.sub.6 to R'.sub.3.75 AlP.sub.3.0 F.sub.4.0 O.sub.9 wherein R' is an alkali metal, and wherein
F:Al is between 2 and 4 PA1 P:Al is between 2 and 3 PA1 R':P is between 0.5 and 1.25.